Processes for forming aldehyde products by the hydroformylation reaction of an olefinic compound with carbon monoxide and hydrogen in the presence of a rhodium-tertiary organophosphine complex hydroformylation catalyst are well know in the art. Of particular interest are those hydroformylation reactions designed to produce aldehydes at low pressures, such as disclosed, e.g. in U.S. Pat. Nos. 3,527,809; 4,148,830 and 4,247,486. Due to the value of the rhodium metal, catalyst lifetime and the ability to reactivate partially deactivated catalysts is of vital importance to the success of commercial hydroformylation.
Experience has shown that, extrinsic catalyst poisons such as halide and carboxylic acid compounds deactivate rhodium - tertiary organophosphine complex hydroformylation catalysts. Such loss in catalytic activity is not to be confused with intrinsic deactivation, i.e., the loss in catalytic activity that inevitably occurs over the course of time during continued prolonged hydroformylation even in the absence of extrinsic poisons. Such intrinsic deactivation is believed to be due to the formation of inactive rhodium complex clusters which are somehow caused by the combined effects of the processing conditions employed. Methods, such as disclosed in U.S. Pat. No. 4,861,918, for reversing such intrinsic deactivation have been found to have little or no effect in reversing catalytic deactivation that has been caused by halide and/or carboxylic acid poisoning. Such extrinsic catalyst poisoning is believed due to the formation of inactive halide-rhodium and/or carboxylic acid-rhodium complexes caused by the presence of such halide and/or carboxylic acids in the hydroformylation reaction medium.
Thus, the presence of such extrinsic poisons in the hydroformylation reaction medium is to be avoided, but such may not always be possible. For instance, such halide or carboxylic acid poisons may enter into the hydroformylation reaction medium as a result of being present as an impurity in one of the reactants, e.g., the olefin feed. Unwanted carboxylic acid might also be present as a result of oxidation of the aldehyde and/or aldehyde condensation by-products during hydroformylation or storage of the reaction medium due to air (i.e., oxygen) contamination. Moreover, such poisons may accumulate over time and can eventually cause the activity of the catalyst to decrease to such a point that it is no longer desirable to operate the hydroformylation process and the catalyst will either have to be reactivated or discharged and replaced with fresh catalyst. Accordingly reactivation of such extrinsically poisoned rhodium complex catalysts is highly important to the state of the art.
U.S. Pat. No. 3,555,098 suggests maintaining or improving the rhodium catalytic activity of a hydroformylation reaction by washing all or a portion of a liquid medium containing the catalyst with an aqueous alkaline solution to remove by-product acid, e.g. carboxylic acid, formed during hydroformylation. However, such a method requires numerous further aqueous washes following the alkaline treatment to ensure complete removal of the alkali employed. Even minor amounts of such alkali compounds left behind in the catalyst solution can strongly catalyze the formation of aldehyde condensation by-products during hydroformylation when using the reactivated catalyst solution and such can be highly detrimental to continuous hydroformylation processes.
Accordingly, there is still a need in the art for a simple method that permits restoration of deactivated rhodium activity that has been caused by halide and/or carboxylic acid poisoning without requiring complicated handling or processing procedures and without introducing unduly adverse side reactions.